In a move that shocked European Union officials, Denmark, citing the need to fight organized crime, unilaterally reintroduced border controls on its land borders with Germany and Sweden on Wednesday. The Danish decision chips away at one of the central principles (and privileges) at the center of the project of the European Union, namely, the free movement of goods, persons, services, and capital.

This principle is put into practice by the Schengen Agreement, which applies to 22 European Union countries as well as four others: Iceland, Liechtenstein, Norway, and Switzerland. (De facto, it also applies to Monaco, San Marino, and the Vatican.) Within this enormous territory, across many national borders, there are no border controls whatsoever.

The Danish government has argued that this decision applies only to border and customs checks and will not extend to passport control, and that it is therefore not a violation of the Schengen Agreement. This argument disingenuously ignores the fact that the border checks will be performed by border control officers who do not have the clearance to act as standard police officers. However this policy change is couched, it amounts to a disengagement with the principles of the Schengen Agreement.

There are also signs that the free movement principle is on the brink of being rethought. At a meeting of EU interior ministers yesterday, several ministers stated their growing willingness to consider rescinding elements of the Schengen Agreement. This movement, spearheaded by the Italian and French foreign ministers, is a response to expectations of a major influx of refugees from North Africa and the Middle East in the coming months.

As it currently stands, EU countries are permitted to introduce border controls in response to discrete events deemed to demand greater security measures. A number of EU countries (Spain, Germany, France, and Finland, among others) have done so at various points over the past decade, but only temporarily.

In the meantime, border control has returned to the Schengen Area. Tourists entering Denmark by land from Germany or Sweden can fairly expect some new scrutiny as long as these checks are enforced.

In the late 1980s, an American spending a summer traveling across Europe with a Eurailpass would see his or her passport stamped possibly dozens of times. With a few exceptions, every time a border was crossed, an immigration agent would pop his or her head into a train compartment, look at everyone’s passports, in most cases stamp them, and move on. Every Eastern Bloc country required visas, some of which could be obtained at the border and others of which had to be applied for in advance.

Today, an American can enter the Schengen zone in Helsinki, fly to Oslo and then on to Amsterdam, proceed by train through Belgium, France, Italy, Slovenia, Austria, Hungary, Slovakia and Poland, then by bus to Lithuania, Latvia, and Estonia, and then by ferry back to Helsinki before catching a flight to Athens and landing in Greece without once needing to submit a passport to a border guard’s scrutiny.

The development of the Schengen agreement across Europe has altered the geopolitical map of the continent in many ways. For tourists, the development of the Schengen zone has simplified travel by drastically reducing the number of times a passport can be checked and stamped as national borders are crossed.

The Schengen Agreement is named after the town of Schengen in Luxembourg. It was here in 1985 that five countries-Luxembourg, Belgium, the Netherlands, West Germany, and France-signed an agreement to essentially create borderless travel between them. A model for this agreement had been created years before by the Benelux countries (Belgium, the Netherlands, and Luxembourg), which eliminated border controls back in 1948. The Nordic countries also did away with internal border posts, in 1958.

In 1995, the five original Schengen countries plus Portugal and Spain inaugurated the zone. In 1997, Austria and Italy joined. Greece followed in 2000 and the five Nordic countries joined in 2001. In late 2007, nine more countries joined the Schengen zone; most recently, Switzerland signed up in 2008.

Abandoned border crossing between Slovakia and Hungary.

Today, 22 European countries are part of Schengen. Every European Union country (save the UK, Ireland, Bulgaria, Romania, and Cyprus) belongs. Other members include EU holdouts Iceland, Norway, and Switzerland. The European microstates present a few complications. Monaco’s borders are administered by France, which makes the tiny principality a part of Schengen, while Liechtenstein’s accession, approved by the European Parliament in February, is pending. San Marino and the Vatican are de facto versus official members, while mountainous, landlocked Andorra remains outside of the zone altogether.

There are five EU countries not currently part of the Schengen zone. The UK and Ireland (as well as the Isle of Man and the Channel Islands) operate a Schengen-like agreement called the Common Travel Area. Neither country is obligated to join the zone.

Romania, Bulgaria, and Cyprus, however, are all bound by treaty to eventually join. Romania has fulfilled all the criteria for joining Schengen and Bulgaria is close to fulfillment as well. These two countries will accede together, likely later this year. Cyprus presents a more complicated situation given the division of the island between the Republic of Cyprus in the south and the largely unrecognized Turkish Republic of Northern Cyprus in the north.

With the coming accession of the Western Balkans to the European Union, the Schengen zone will almost definitely continue to grow. Might it one day cover the entire landmass of Europe? Check back in two decades.